This invention relates to load pull testing of microwave power transistors using automatic microwave tuners for synthesizing reflection factors (or impedances) and match the transistors (device under test, or DUT) at the input and output of the DUT's at the fundamental and harmonic frequencies.
Modern design of high power microwave amplifiers, oscillators and other active components used in various communication systems requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient for the transistors operating at high power close to saturation, to be described using analytical or numerical models. Instead the devices need to be characterized using specialized test setups using tuners and other test equipment under the actual operating conditions.
An efficient method for testing and characterizing transistors for high power operation is “load pull” and “source pull”. Load pull or source pull are measurement techniques employing microwave impedance tuners and other microwave test equipment (FIG. 1), such as signal sources, input and output impedance tuners, RF (Radio Frequency) load, control computer and digital connections between the computer and the tuners and the test equipment. The microwave tuners in particular are used in order to manipulate the RF impedance conditions at the fundamental (Fo) and harmonic (2Fo, 3Fo and possibly higher) frequencies under which the Device under Test (DUT) is tested.
Electromechanical tuners used in load pull measurements are so called “slide screw tuners” using a low loss transmission line, typically a slotted airline (slabline) and one or more metallic probes (see references 1, 2 and 3) that can be precisely inserted into the slot of the slabline in order to create a controlled reflection factor, or RF impedance (FIGS. 3, 4). Electro-mechanical slide screw tuners are used for high power load pull testing, because they have several advantages, such as long-term stability, higher handling of RF power, easier operation and lower cost, compared to other types of tuners.
It has been found that multi-probe tuners (see references 2 and 3) can synthesize simultaneously and independently RF impedances at more than one harmonic frequency (see reference 4). In order to do so such tuners must be calibrated before-hand on a vector network analyzer and a search and tune algorithm is needed to be able to select probe settings to allow for this to happen. It is the aim of this invention to describe such an algorithm.